Objective: The primary aim of this study was to compare the acute effects of a caffeine based supplement on the strength performance of trained and untrained individuals with a secondary investigation into the effects of a placebo.Method: Seven resistance trained (>6 months) and seven untrained (<6 months) males (mean ± SD: age: 21 ± 3 y, mass: 75.2 ± 11.3 kg, height: 176 ± 6 cm) consumed either caffeine (CAF) (5 mg.kg.bw -1 ), placebo (PLA) or nothing (CON) 60 minutes prior to 1 RM squat measurements in a double-blinded, repeated measures design. A two way repeated measures ANOVA was applied to test for the main effects of condition (CAF, PLA, CON) and group (Trained, Untrained), and the interaction effect (condition x group).Results: A significant interaction effect (F (2,11) =4.38, p=0.024) for 1 RM was observed. In the untrained group there was significant difference between CON and PLA (p<0.001). On average 1 RM in the untrained group was 12% lower in the CON trial (92.1 kg) compared to the PLA (102.9 kg; 95% CI=-5.3 to -16.1 kg), and 9% lower compared to CAF (p=0.005; 95% CI=-2.7 to 14.5 kg). There was no significant difference in 1 RM in the untrained group between PLA and CAF (p=0.87, 95% CI -3.2 to 7.5 kg). Additionally, there were no significant differences for the trained group between conditions. There was also a significant main effect for condition for 1 RM (F (2,11)= 12.81, p<0.001) . Overall the CON trial was 6% lower (p=0.001, 95% CI=-3.0 to -10.6 kg) than the PLA trial (117.9 kg; 95% CI 97.6 to 124.6 kg), and 5% lower (p=0.12, 95% CI=-1.2 to -9.5 kg) than the CAF trial (116.4 kg; 95% CI 105.0 to 127.8 kg). There was no significant difference between PLA and CAF (p=0.951). Finally, there was a significant main effect for group (F (1,12) =8.79, p=0.12). On average 1 RM was 25% higher in the trained group (131.7 kg; 95% CI=114.5 to 148.9 kg) compared to the untrained group (98.6 kg; 95% CI=81.4 to 115.8 kg). Conclusion:These findings suggest that both a caffeine supplementation and placebo improve 1 RM in untrained individuals but do not improve performance in resistance trained athletes. No significant differences between caffeine and placebo, suggests placebo induced mechanisms also need to be considered.

Caffeine (1, 3, 7-trimethylxanthine) which can be ubiquitously found in energy drinks, sodas, coffee, and supplements, is one of the principal legal drugs consumed worldwide. Caffeine based ergogenic aids are utilized prolifically within training and competition for an ergogenic benefit to enhance sporting performance by both recreational and elite athletes. The evidence of caffeine's ability to enhance endurance performance is well established, however, evidence of an ergogenic benefit for muscular endurance and strength-based tasks is limited. Moreover, the limited evidence for caffeine's ergogenic benefit in muscular endurance and strength is equivocal, and therefore, practical recommendations for the implementation of caffeine supplementation in training and competition for coaches, and practitioners is difficult. Indeed, it is currently not known if, and how caffeine may improve muscular endurance and/or strength based tasks. Variability in the findings could be due to several factors including muscles tested, participant characteristics, exercise protocol, type and dose of caffeine used. This brief review will discuss the current literature relating to the potential efficacy of caffeine to enhance muscular endurance and strength based performance, and provides evidence based recommendations for athletes and coaches to implement. Caffeine's primary mechanistic process likely occurs through the antagonising of adenosine [27]. This process is achieved by caffeine binding to adenosine receptors, reducing adenosine's ability to slow neural activity, reduce arousal, and induce sleep [28]. Further rewards from caffeine's effect on adenosine receptors include enhanced neurotransmitter release, increased firing rates, and amplified spontaneous and evoked potentials [29]. Caffeine has also been shown to alter metabolic substrate utilization [9] and provide enhanced fat oxidation and consequential glycogen sparing [30]. Alterations to pain perception following caffeine supplementation have also been reported [31] most likely due to enhanced secretion of β-endorphins [32]. More specifically to strength performance, possible mechanisms also include increased muscle activation [33], motor unit recruitment [34], and enhanced excitation contraction coupling [35]. KeywordsIt should be noted, that it is beyond the scope of this review to provide a comprehensive overview of the mechanisms of caffeine. The primary purpose of this review is to provide in depth analysis of the evidence relating to the use of caffeine in muscular endurance and strength based exercise, and provide coaches and athletes, at both elite and recreational level, with recommendations for the use of caffeine with regards to muscular strength and muscular endurance. For a complementary, wider-ranged review of all current literature, readers is direct to other published review articles [36,37] and a meta-analysis [38].